In the shrinking of biaxially stretched film webs, it is possible to take advantage of the difference in length between a film web that already enters the treatment apparatus in corrugated wavy form and a film web that has a plane configuration as it enters the treatment apparatus. In the first instance where the entering web is corrugated or wavy, substantial difficulties have been encountered in the feeding of the corrugated film web into the entrance zone or area of the apparatus. On the one hand, the tentering clamps must have a configuration that takes the wavy shape of the film web into account. On the other hand, the tentering chain travelling speed must be smaller than the web feed-in speed in order to take into account the intended shrinking distance.
The above mentioned U.S. Ser. No. 07/915,000 (Rutz et al.), filed on Jul. 16, 1992, discloses a tentering chain with tentering clamps which are so tiltable that the clamps can clamp a corrugated film web. U.S. Ser. No. 07/915,416 (Rutz) filed Jul. 17, 1992 discloses a tentering chain with a variable pitch between neighboring tentering clamps.
In the second instance where the film web enters the treatment apparatus as a plane sheet, the film web must be corrugated or brought into a wavy shape within the treatment apparatus. The wavy shape is formed in the heated zone of the apparatus by an additional longitudinal stretching without changing the film web characteristics. In this approach there are no difficulties in feeding the plane film web into the apparatus. However, the application of the required stretching forces to the tentering clamps as they travel through the heating zone of the apparatus poses certain difficulties. U.S. application Ser. No. 07/915,000 (Rutz et al), on Jul. 16, 1992 discloses solutions for these problems.
The tiltable tentering clamps of U.S. Ser. No. 07/915,000 are even suitable for use in those instances where a plane film web needs to be fed into the treatment apparatus with a small pulling-in speed in the inlet zone of the apparatus. In U.S. Ser. No. 07/915,000 an incoming plane film web is gripped in the inlet zone of the apparatus by tentering clamps which have a clamp table and a clamp flap extending in parallel to the plane film web in the inlet zone. These tentering clamps are variable in their position for forming the corrugation. A pair of tentering clamps grips the edges of the entering film web between the table and flap and the clamps are caused to perform a motion when the clamping action is completed and even before the next pair of clamps grips the film edges further upstream of the web. The motion performed by the clamps or at least by the table and flap is preferably a tilting motion around an axis extending in parallel to the film travel direction. This procedure, however, is possible only if the tentering chains travel with a certain optimal speed which may not be too fast, because the control mechanism for the tilting of the tentering clamps must work very fast. Thus, at high speeds of the tentering chains, the control mechanisms for the clamp tilting are not quite suitable, since their required fast operation poses certain mechanical problems. Conventional tentering clamp guides which permit a shrinking of the film web by way of a pitch change of the pitch between neighboring tentering clamps on a tentering chain permit the feeding of the film web into the treatment apparatus without any problems. Incidentally, the term "pitch" as used herein means the on-center spacing between two neighboring tentering clamps along a tentering chain. However, tentering clamps that permit varying the pitch do not permit a positional change of the clamps in directions other than the travel direction. The pitch is changed in the travel direction. Further, problems occur in connection with the constructions that permit a pitch change due to the relatively high adjustment forces in the travel direction that are necessary when a change is made from a normal pitch T to the shrinking pitch t. This problem is aggravated by the fact that the pitch change must take place in areas along the tentering chain where the tentering chain is exposed to the maximum loads along its travelling length. Thus, all adjustment forces required for the pitch change must also be taken up by the adjustment mechanism for the pitch change. These adjustment mechanisms either are effective on the respective tentering clamp body or between two neighboring tentering clamp bodies along the tentering chain.